Of course, you could have used the two N++ built-in commands Edit > Blank operations > EOL to Space or Edit > Blank operations > Remove Unnecessary Blank and EOL, but :

The later command does not merge consecutive Line Break, before changing it by a single Space character

The former command acts as the later command and, in addition, does not trim any trailing blank character ( Space or Tab ), before replacing it by a single Space character

On the contrary, my regex will replace, systematically, any gap between two non-empty lines, with a single Space character :-)) Saying "non-empty lines", I mean lines containing, at least, one non-blank character !

Now, if your text contains some words, separated with more than 1 space or with Tabulation characters and than you would like to normalize all these blank characters, between words, to a single Space character, use the following regex S/R :